A current mirror, as its name suggests, is utilized in integrated circuits to mirror (e.g., copy) a reference current flowing through one active device (e.g., transistor) in another active device (e.g., another transistor). The current mirror is intended to maintain the output current (e.g., the mirrored reference current) at a constant level regardless of load changes at the other active device. Further, the current being mirrored can be a direct current (“DC”) or an alternating current (“AC”). Current mirrors are generally utilized in integrated circuits to provide bias currents and/or active loads.
However, current mirrors are still susceptible to errors. For example, in many current mirrors, the current source transistor (e.g., the transistor utilized to generate the reference current) has low output impedance, leaving the current source transistor more sensitive to noise in the integrated circuit. For example, the current source transistor is less able to reject noise from a power supply when the output impedance is low. Further, the low output impedance also leads to a lower power supply rejection ratio (“PSRR”). The PSRR is a ratio of the change in supply voltage to the change in output voltage. As such, as the power supply modulates (e.g., due to noise), so will the VDS across the current source transistor and, therefore, the current generated by the current source transistor. As the reference current modulates, it becomes more difficult to effectively maintain a desired ratio of the output current to the reference current.
Further, the power requirements for the current mirror transistor and the output current transistor also affect the performance of the current source transistor. For example, both of the current mirror transistor and the output current transistor have to operate in the saturated region in order to mirror the reference current at the output current transistor. Therefore, the VDS of each transistor has to be greater than the difference between the VGS and VT of the transistor (e.g., VDSat>VGS−VT). As the VDS for the transistors modulates, so will the VDS across the current source transistor, which, as stated above, makes it more difficult to maintain the desired ratio of the output current to the reference current.
In order to address the aforementioned effects of low output impedance, many current mirrors include a cascode transistor at the drain node of the current source transistor. The cascode transistor is essentially a gain amplifier that amplifies (e.g., multiplies) the low output impedance at the drain node of the current source transistor, resulting in a higher output impedance. Further, the cascode transistor disjoins the dependency of the VDS voltage across the current source transistor from (i) the power supply and (ii) the VDS voltage across the current mirror transistor. Therefore, any voltage modulations (e.g., due to noise or otherwise) from the power supply or the current mirror transistor would not affect the VDS voltage across the current source transistor, thereby maintaining the desired ratio of the output current to the reference current. However, in circuit designs with low power supply requirements, there may not be enough headroom (e.g., remaining voltage) to include the cascode transistor.
Noise in the system can be further mitigated by increasing the VDSat of the current mirror transistor and the output current transistor. However, as the VDSat of the current mirror and output current transistors are increased, less headroom will be available for the current source transistor. Further, if (i) the VDS across the transistors (e.g., current mirror and output current) are high and (ii) the reference current is also high, the reference current will likely compress. As the reference current compresses, it will again become more difficult to effectively maintain the desired ratio of the output current to the reference current.
Accordingly, there is a need for a low-noise current mirror to operate under low power supply requirements.